feat(gameplay): implement in-system onboarding with docked station view

Implement the final onboarding step where the player loads into their
selected starting system docked at a station.

New features:
- Create in_system module for system-scale gameplay
- Spawn player ship docked at highest-population station
- Display station info panel with undock button
- Position camera for cinematic docked view with orbit controls

Implementation details:
- in_system/mod.rs: Plugin setup with DockedState and ActiveSystem resources
- in_system/scene.rs: System/POI spawning and player ship docked positioning
- in_system/docked.rs: Docked state management and UndockEvent
- in_system/ui.rs: Docked UI with station details and undock button
- Reuse existing POI spawning patterns from galaxy/contents.rs
- Select docking station by highest population (better for new players)

Modified files:
- Add in_system module exports to gameplay/mod.rs
- Register InSystemPlugin in main.rs
- Update orbit camera control for InGame state
- Re-export GeneratedStation and STARTING_SHIPS for use by in_system

The player now completes onboarding by loading into a system view with
their ship docked at a station, ready for gameplay.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/game/src/gameplay/in_system/mod.rs

@@ -0,0 +1,46 @@
+//! In-system gameplay module.
+//!
+//! Handles the system-scale view where the player is docked at a station
+//! within their selected starting system. This is the entry point to actual
+//! gameplay after onboarding.
+
+mod docked;
+mod scene;
+mod ui;
+
+use bevy::prelude::*;
+
+use crate::state::AppState;
+
+pub use docked::{DockedState, UndockEvent};
+pub use scene::ActiveSystem;
+
+pub struct InSystemPlugin;
+
+impl Plugin for InSystemPlugin {
+    fn build(&self, app: &mut App) {
+        app.init_resource::<DockedState>()
+            .init_resource::<ActiveSystem>()
+            .add_event::<UndockEvent>()
+            .add_systems(
+                OnEnter(AppState::InGame),
+                (scene::setup_in_system_view, ui::setup_docked_ui).chain(),
+            )
+            .add_systems(
+                OnExit(AppState::InGame),
+                (
+                    ui::despawn_docked_ui,
+                    scene::despawn_in_system_scene,
+                ).chain(),
+            )
+            .add_systems(
+                Update,
+                (
+                    ui::refresh_docked_ui,
+                    ui::undock_button_handler,
+                )
+                    .chain()
+                    .run_if(in_state(AppState::InGame)),
+            );
+    }
+}